A cellular telecommunications system is based around cells or similar radio access and/or service areas. Examples of cellular telecommunications systems include standards such as the GSM (Global System for Mobile communications) or various GSM based systems (such as GPRS: General Packet Radio Service), AMPS (American Mobile Phone System) or DAMPS (Digital AMPS) or WCDMA (Wideband Code Division Multiple Access) and TDMA/CDMA (Time Division Multiple Access/Code Division Multiple Access) in UMTS (Universal Mobile Telecommunications System), IMT 2000 and so on.
In cellular systems, a base transceiver station (BTS) serves mobile stations (MS) or similar wireless user equipment (UE) via an air or radio interface. A base station provides a radio access entity that is typically but not exclusively referred to as a cell. The approximate location, size and shape of a cell is known, each of the cells covering a particular geographical area. The size and shape of the cells may vary from cell to cell. Several cells may also be grouped together to form a larger service area. Each of the cells can be controlled by an appropriate controller apparatus. The various controllers of a cellular network may be interconnected and there may be one or more gateway nodes connecting the cellular network e.g. to a public switched telephone network (PSTN) and other communication networks such as to the Internet and/or other packet switched networks. A geographical area may be served by more than one base stations. The mobile station may also be in communication with two or more base stations of the system at the same time. The two or more base stations may be connected to the same controller or different controllers.
The cellular communication network apparatus can also be employed for provision of location information concerning a mobile station and the user thereof. More particularly, the cells or similar geographically limited radio access entities facilitate the cellular telecommunications system to produce at least a rough location information estimate concerning the current geographical location of a mobile station, as the cellular telecommunications system is aware of the cell with which a mobile station currently associates. Therefore it is possible to conclude from the location of the cell the geographical area in which the mobile station is likely to be at a given moment. This information is available also when the mobile station is located within the coverage area of a visited or “foreign” network. The visited network may be capable of transmitting location information of the mobile station back to the home network, e.g. to support location services or for the purposes of call routing and charging.
The management of the location service may be provided by a separate network element such as a location server which is adapted to receive information from the telecommunication system.
The location information may be based on information of e.g. the current cell identity. If no further computations and/or approximations are made, this would give the location to an accuracy of one cell, i.e. it would indicate that the mobile station is (or at least was) within the coverage area of a certain cell. However, more accurate information concerning the geographical location of a mobile station may be desired. For example, the United States Federal Communication Commission (FCC) has mandated that wireless service providers have to implement location technologies that can locate wireless phone users who are calling to E911 emergency centre. The FCC has also defined certain accuracy requirements for the location.
Although the FCC order is directed to emergency caller location, other (commercial and non-commercial) uses for mobile systems, such as fleet management, location-dependent billing and navigation, and various advertisement schemes might also find more accurate location information useful. The possible applications include different local advertisement and information distribution schemes (e.g. transmission of information directed to those mobile users only who are currently within a certain area), area related WWW-pages (such as time tables, local restaurant, shop or hotel guides, maps local advertisements etc.) for the users of mobile data processing devices, and tracking of mobile users by anyone who wishes to receive this information and is legally entitled to obtain it. It should be appreciated that the above are only examples and there are several other possible commercial and non-commercial applications which may use the location information provided by the location service (LCS).
More accurate location information may be obtained e.g. by calculating the geographical location from range measurements or range difference (RD) measurements. More particularly, the reliability of the location determination may be improved by utilising results of measurements which define the travel time (or travel time differences) of the radio signal sent by the mobile station to the base station. The measurements are accomplished by a number (preferably at least three) base stations covering the area in which the mobile station is currently located. The measurement by each of the base stations gives the distance (range) between the base station and the mobile station or distance difference (range difference) between the mobile station and two base stations. Possible measurement errors may be corrected by an appropriate method. It should be appreciated that other measurements may also form the base for the location estimation.
Each of the range measurements generates a circle that is centred at the measuring base station. The mobile station can then be determined to be located at an intersection of the circles. The range difference measurement by two base stations creates a hyperbola. Observed time difference (OTD), E-OTD (Enhanced OTD) and time difference of arrival (TDOA) are mentioned herein as examples of technologies that are based on RD measurements.
In the E-OTD location method the mobile station (MS) measures the Observed Time Difference (OTD) between the arrivals of bursts from the serving (BTS 1) and neighbour base stations (BTS 2). The OTD value consists of two components:OTD=RTD+GTD  [1]
In equation [1] the RTD (Real Time Difference) is the synchronisation difference between two base stations. It describes how much earlier or later a base station transmits than another base station. If the network is synchronised, the RTDs should be zero. The GTD (Geometric Time Difference) is the component that is due to different propagation times (i.e. distances) between the mobile station MS and the two base stations. This is the actual quantity that includes information about the location:GTD=[d(MS,BTS2)−d(MS,BTS1)]/c  [2]where
d(MS,BTSx) is the distance between the MS and BTS x, and
c is the speed of light.
The above equation [2] determines a hyperbola, which is the curve of possible locations for a mobile station MS observing a constant GTD value between the two base stations BTS at known positions. When there are at least two such hyperbolas available (i.e. one serving and two neighbouring BTS sites are used for the measurements), the location estimate can be found at the intersection of the hyperbolas (see FIG. 1). If more E-OTD values are available then the possible location area can be more accurately be deduced.
A cellular network, such as for example a GSM network may not be synchronised. Thus the Real Time Difference (RTD) between the different base stations must be known in order to be able to locate a mobile station MS based on E-OTD measurements. The RTD values are typically measured by E-OTD Location Measurement Units (E-OTD LMU) located in the network. Therefore an implementation of an Enhanced Observed Time Difference (E-OTD) method for the Location Services (LCS) in a GSM network may require use of Location Measurement Units (LMUs) or similar entities. The LMU may be positioned either independently from the base station sites or co-site with a base station BTS.
The right positioning of the LMUs may have a significant effect on the performance of the LCS system and thus the positioning of the LMUs has to be carefully planned. The work of LMU position planning can be done by network planning tools during the network planning phase.
A cellular telecommunications network typically includes hundreds or even thousands of base station sites. Therefore it may be difficult to do the LMU position planning manually. It may also be very difficult to get the optimised positions for the LMU sites. The implementation and running of the LMUs cause costs for the operators of the communication networks. Therefore it could be advantageous to be able to minimise the amount of the LMUs in the network. Hence an automatic and/or optimised LMU position planning process might be useful.